Symbology

In a Fault Tree, two types of nodes are usually used: primary events and logic gates.

The primary events are failure events that are expected to occur with a given probability. The logic gates descibe the Boolean relationship beetween the primary events or other logic gates.

The primary events symbols can be the following:

Primary Event	Symbol	Description
Basic		A basic event represents the lowest level in a fault tree. A basic event has no inputs. It simply represents the occurrence of an event in the system being analyzed.
Basic Repeated		A repeated event is a single event that affects multiple portions of a fault tree. When a repeated event occurs, it occurs simultaneously everywhere it is inserted in the tree.
Basic Dormant		A dormant event is a basic event with no identifiable means of detecting the specific failure mode during normal operation, until it causes an evident failure or there is a maintenance action. The calculated probability is based on the applicable maintenance interval.
Basic Repeated Dormant		A dormant event repeated. See repeated event.
Basic Hidden		A hidden event is a basic event that is not detectable neither as it occurs nor later, in any normal operation, neither during maintenance. No identifiable means of detecting until it will cause an evident failure. The calculated probability is based on the system life hours.
Basic Repeated Hidden		A hidden event repeated. See repeated event.
Undeveloped		An undeveloped event is used when further resolution of the event is not necessary or when the information for developing the event is not currently available. While an undeveloped event is sismilar to a basic event, it is represented by a different symbol to indicate that is possible to break this event down into associated gates and events.
Undeveloped Repeated		An undeveloped event repeated. See repeated event.
Undeveloped Dormant		An undeveloped event with dormant failure. See Basic Dorment.
Undeveloped Repeated Dormant		An undeveloped event with dormant failures repeated.
Undeveloped Hidden		An undeveloped event with hidden failures. See Basic Hidden.
Undeveloped Repeated Hidden		An undeveloped event with hidden failures repeated.

The logic gate symbols are the followings:

Gate	Symbol	Description	Gate logic
AND		The AND gate is used to indicate that the output occurs if and only if all the inputs events occurs.	All events must be TRUE for the output to be TRUE
OR		The OR gate is used to indicate that the output occurs if any of the input events occurs.	If at least one event is TRUE, the output is TRUE. If all events are FALSE, the output is FALSE.
Transfer		A transfer gate is used to indicate continuation of the fault tree in a separate area.	N/A
Remark		A remark gate is used to add relevant information without being involved in the FTAs calculation.	N/A